1. Field of the Invention
The present invention relates to analysis of organometallic compounds, and more particularly, analysis of organometallic compounds which are air or moisture sensitive, or pyrophoric.
2. Description of the Prior Art
Metal alkyls of Groups II, III and IV are used in combination with metal hydrides and alkyls of Groups V and VI for the formation of semiconductor materials and alloys by means of chemical vapor deposition. The purity levels of these highly reactive organometallic compounds is of primary importance, because contaminating elements in the .mu.g/g range may completely alter the properties of the semiconductor materials formed.
However, the extreme reactivity of these organometallic compounds makes analysis of trace impurities very difficult. For example, trimethylgallium is typical in that it is a liquid which reacts pyrophorically with air and moisture. Thus, analysis of such a compound must be performed under an inert atmosphere or the compound must be decomposed prior to analysis. Both of these methods have been used in the prior art with limited success.
Decomposition methods are typically directed to forming oxides of the compound and any impurities. One procedure for decomposing trimethylgallium (TMG) is to add a small volume of TMG to several times that volume of hexane, and then decompose the TMG behind a blast shield in a fume hood. Distilled deionized water is added dropwise until about twice as much water by volume has been added as the original volume of TMG. A heat gun is used to evaporate the hexane. When the hexane vapor is replaced by water vapor, the heat gun is replaced by a torch and the sample is heated at a temperature of about 300-400.degree. C. until a free flowing oxide is generated. This oxide is then analyzed by various well known techniques, such as direct current arc emission spectroscopy.
An alternative decomposition procedure is to simply ignite a small aliquot of TMG behind a blast shield in a fume hood. After each aliquot has ceased burning, an additional aliquot is treated similarly until a sufficient sample of oxide is obtained for analysis.
Both of the above decomposition methods suffer from the limitation that the heat of oxide formation results in the loss of most of any volatile oxides which are generated, as well as the possibility that materials which oxidize slowly will be vaporized prior to conversion to oxide. These limitations are particularly problematical when it is considered that the more volatile impurities are more likely to be incorporated into the product during chemical vapor deposition.
An alternative method has attempted determination of impurities by direct analysis of the organometallic by dissolution in a suitable organic solvent (e.g., methyl isobutyl ketone, xylenes, methanol/ethanol, or toluene), followed by nebulization and analysis by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).
A nebulizer is used to mix the sample to be analyzed with a suitable gas, e.g., argon. As the sample is ejected from the outlet of the nebulizer, discrete droplets are obtained which continue within the gas stream into the ICP unit. Only about 5-10% of the liquid which enters the nebulizer forms individual droplets of a size that they are carried into the ICP unit. It has been assumed that the remainder of the liquid simply falls out of the stream entering the ICP unit where it passes into a drain and is collected.
This latter method suffers from the disadvantage that it must be assumed that the droplets entering the ICP unit are representative of the sample being tested, and this is not the case, especially when organic solvents are being used for the analysis of TMG. It must also be assumed that none of the solvent or analyte vapors enter the ICP except as droplets.
Accordingly, a need existed for a method of analyzing air or moisture sensitive or pyrophoric organometallic compounds without the concomitant disadvantages of the prior art.